U.S. patent number 6,833,207 [Application Number 09/986,635] was granted by the patent office on 2004-12-21 for unitized regenerative fuel cell with bifunctional fuel cell humidifier and water electrolyzer.
This patent grant is currently assigned to Hydrogenics Corporation. Invention is credited to Joseph Cargnelli, David Frank, Nathanial Ian Joos.
United States Patent |
6,833,207 |
Joos , et al. |
December 21, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Unitized regenerative fuel cell with bifunctional fuel cell
humidifier and water electrolyzer
Abstract
A regenerative fuel cell system, comprising an electrolyzer
portion and a fuel cell portion; the electrolyzer portion has a
closeable hydrogen inlet and a hydrogen outlet in communication
with the cathode of the electrolyzer portion for conducting
hydrogen, a gas bypass having a gas bypass inlet and a gas bypass
outlet for conducting oxidant gas for fuel cell reaction to the
fuel cell portion, a water inlet and an oxygen-water outlet for
exhausting oxygen generated in electrolyzer operation and coolant
water from the fuel cell portion out of the electrolyzer portion;
the fuel cell portion has a hydrogen inlet, a first closeable
hydrogen outlet for exhausting excess hydrogen in fuel cell mode, a
second closeable hydrogen outlet for exhausting hydrogen generated
in the electrolyzer portion in electrolyzer mode, an oxidant gas
inlet, an oxidant gas outlet, a coolant water inlet and a coolant
water outlet; and the hydrogen inlet of the fuel cell portion being
in communication with the hydrogen outlet of the electrolyzer
portion; the oxidant gas inlet of the fuel cell portion being in
communication with the gas bypass outlet of the electrolyzer
portion; and the water inlet of the electrolyzer portion being in
communication with the coolant water outlet of the fuel cell
portion.
Inventors: |
Joos; Nathanial Ian (Toronto,
CA), Frank; David (Scarborough, CA),
Cargnelli; Joseph (Toronto, CA) |
Assignee: |
Hydrogenics Corporation
(Mississauga, CA)
|
Family
ID: |
25532609 |
Appl.
No.: |
09/986,635 |
Filed: |
November 9, 2001 |
Current U.S.
Class: |
429/415;
204/DIG.4; 429/417; 429/422; 429/456; 429/515 |
Current CPC
Class: |
H01M
8/0656 (20130101); H01M 8/04029 (20130101); H01M
8/186 (20130101); H01M 8/04119 (20130101); Y02E
60/528 (20130101); Y10S 204/04 (20130101); Y02E
60/50 (20130101) |
Current International
Class: |
H01M
8/18 (20060101); H01M 8/06 (20060101); H01M
8/04 (20060101); H01M 008/18 () |
Field of
Search: |
;204/DIG.4
;429/13,17,19 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chaney; Carol
Attorney, Agent or Firm: Bereskin & Parr
Claims
What is claimed is:
1. A regenerative fuel cell apparatus comprising an electrolyzer
portion and a fuel cell portion; wherein the electrolyzer portion
has a cathode including a first electrolyzer cathode port and a
second electrolyzer cathode port, and an anode including a first
electrolyzer anode port and a second electrolyzer anode port;
wherein the fuel cell portion comprises a fuel cell anode including
a first fuel cell anode port and a second fuel cell anode port, a
fuel cell cathode including a first fuel cell cathode port and a
second fuel cell cathode port, and at least one coolant channel
including a first coolant port and a second coolant port; and
wherein the regenerative fuel cell system includes at least one of:
(a) a connection between the second electrolyzer cathode port and
the second fuel cell anode port, thereby to provide a continuous
passage between the first electrolyzer cathode port and the first
fuel cell anode port for hydrogen, through the electrolyzer cathode
and the fuel cell anode, (b) a connection between the second
electrolyzer anode port and the second coolant port, thereby to
provide a continuous passage between the first electrolyzer anode
port and the first coolant port through the electrolyzer anode and
said at least one coolant channel; and (c) within the electrolyzer
portion a gas bypass conduit including a first gas bypass port and
a second gas bypass port, a connection between the second gas
bypass port and the second fuel cell cathode port, thereby to
provide a continuous passage between the first gas bypass port and
the first fuel cell cathode port through bypass port conduit and
the fuel cell cathode.
2. A regenerative fuel cell apparatus as claimed in claim 1, which
includes both the connection between the second electrolyzer
cathode port and the second fuel cell anode port, and the
connection between the second electrolyzer anode port and the
second coolant port.
3. A regenerative fuel cell apparatus as claimed in claim 2, which
additionally includes the connection between the second gas bypass
port and the second fuel cell cathode port, and the gas bypass
conduit with the first and second gas bypass ports.
4. A regenerative fuel cell apparatus as claimed in claim 3,
wherein the fuel cell portion and the electrolyzer portion are
juxtaposed.
5. A regenerative fuel cell apparatus as claimed in claim 4,
wherein the fuel cell portion and the electrolyzer portion are
integral with one another, and wherein the fuel cell portion and
the electrolyzer portion each include end plates, the end plates
facing one another, wherein an end plate of the fuel cell portion
adjacent an end plate of the electrolyzer portion includes said
second ports.
6. A regenerative fuel cell apparatus as claimed in claim 5, which
includes an insulator between said pair of adjacent end plates.
7. A regenerative fuel cell apparatus as claimed in claim 5,
wherein the end plates of the fuel cell portion and the
electrolyzer portion are integral and are provided as a common
separator plate, including said second ports.
8. A regenerative fuel cell apparatus as claimed in claim 6 or 7,
wherein the electrolyzer portion comprises a plurality of
individual cells each including a membrane exchange assembly, an
anode flow field plate and a cathode flow field plate, wherein the
electrolyzer anode comprises said anode flow field plates and the
electrolyzer cathode comprises said cathode flow field plates, and
wherein the gas bypass conduit of the electrolyzer comprises a
plurality of bypass conduits, each extending between an adjacent
pair of electrolyzer cells.
9. A regenerative fuel cell apparatus as claimed in claim 8,
wherein the fuel cell portion comprises a plurality of individual
fuel cells, each comprising a membrane exchange assembly, an anode
flow field plate and a cathode flow field plate, wherein the fuel
cell anode comprises said anode flow field plates of the fuel cells
and the fuel cell cathode comprises said cathode flow field plates
of the fuel cells, and wherein said at least one coolant channel
comprises a plurality of coolant channels provided between adjacent
pairs of fuel cells.
10. A regenerative fuel cell apparatus as claimed in claim 9,
wherein the electrolyzer portion is provided above the fuel cell
portion.
11. A regenerative fuel cell apparatus as claimed in claim 9,
wherein the fuel cell portion includes a third fuel cell anode
port, and a conduit providing a direct passage between the second
and third fuel cell anode ports, whereby, in use, in a fuel cell
mode of operation, hydrogen gas passes between the first and second
fuel cell anode ports and across the individual fuel cell anode
flow field plates, and in an electrolyzer mode of operation, the
generated hydrogen can pass between the first and third fuel cell
anode ports.
12. A regenerative fuel cell apparatus as claimed in claim 4 or 5,
wherein the first electrolyzer cathode port is closable, whereby in
the electrolyzer mode of operation, hydrogen is withdrawn through
the fuel cell portion.
13. A regenerative fuel cell apparatus as claimed in claim 11,
wherein the first electrolyzer cathode port is closable, whereby in
the electrolyzer mode of operation, hydrogen is withdrawn through
the fuel cell portion.
14. A regenerative fuel cell apparatus as claimed in claim 4 or 5,
wherein the first electrolyzer cathode port is adapted to withdraw
hydrogen from the electrolyzer in the electrolyzer mode, and
wherein means are provided for purging water from the cathode of
the electrolyzer in the electrolyzer mode.
15. A regenerative fuel cell apparatus as claimed in claim 14,
wherein said means for purging water comprises a purge valve
connected to the second electrolyzer cathode port, whereby, in use,
with the first electrolyzer cathode port oriented above the second
electrolyzer cathode port, hydrogen is withdrawn from the first
electrolyzer cathode port and water is purged from the electrolyzer
cathode through said purge valve.
16. A regenerative fuel cell apparatus as claimed in claim 12,
which includes a valve provided between the second electrolyzer
cathode port and the second fuel cell anode port and wherein a
discharge line for hydrogen is provided connected to the valve,
whereby, in use, the valve connects the second electrolyzer cathode
port to the second fuel cell anode port for supply of hydrogen to
the fuel cell portion in the fuel cell mode, and, in an
electrolyzer mode of operation, the valve connects the second
electrolyzer cathode port to the discharge line, for discharging
generate hydrogen and any entrained water.
17. A regenerative fuel cell apparatus comprising an electrolyzer
portion and a fuel cell portion, wherein the electrolyzer port and
the fuel cell portion are integral with one another, and wherein
there is at least one passage for a fluid extending through both of
the electrolyzer portion and the fuel cell portion and including
one connection port on the electrolyzer portion and another
connection port on the fuel cell portion.
18. A regenerative fuel cell apparatus as claimed in claim 17,
wherein the electrolyzer portion and the fuel cell portion have
similar cross-sections and include common clamping elements
securing the regenerative fuel cell apparatus together.
19. A regenerative fuel cell apparatus as claimed in claim 18,
wherein the electrolyzer portion comprises a plurality of
individual cells each including a membrane exchange assembly, an
anode flow field plate and a cathode flow field plate, wherein the
anode flow field plates form an electrolyzer anode, the cathode
flow field plates form an electrolyzer cathode, and wherein a
plurality of gas by-pass conduits are provided extending between
the individual cells.
20. A regenerative fuel cell apparatus as aimed in claim 19,
wherein the fuel cell portion comprises a plurality of individual
cells, each comprising a membrane exchange assembly, an anode flow
field plate and a cathode flow field elate, wherein the anode flow
field plates provide the fuel cell anode, the cathode flow field
plates provide the fuel cell cathode and wherein a plurality of
coolant channels are provided extending between adjacent pairs of
fuel cells.
21. A regenerative fuel cell apparatus as claimed in claim 20,
wherein the electrolyzer portion includes: a first electrolyzer
cathode port for hydrogen, a second electrolyzer cathode port, a
first electrolyzer anode port for water and, in the electrolyzer
mode, water, a second electrolyzer anode port, a first gas by-pass
port for an oxidant and a second gas by-pass port; wherein the fuel
cell portion includes a first fuel cell anode port for hydrogen and
a second anode fuel cell port connected to the second electrolyzer
cathode port, a first fuel cell cathode port for an oxidant and a
second fuel cell cathode port connected to the second gas by-pass
port, a first coolant port and a second coolant port connected to
the second electrolyzer anode port, for passage of water as a
coolant.
22. A method of operating a regenerative fuel cell apparatus
including an electrolyzer portion and a fuel cell portion, the
method comprising effecting, alternately: a) in a fuel cell mode of
operation, supplying hydrogen and an oxidant to the fuel cell to
generate electricity, withdrawing water from the fuel cell and
passing a coolant through the fuel cell, wherein the method
includes passing at least one of the fuel gas, the oxidant and the
water through the electrolyzer portion; and b) in an electrolyzer
mode of operation, supplying water to the electrolyzer and electric
current to electrolyzer water to generate oxygen and hydrogen and
withdrawing oxygen, hydrogen and residual water from the
electrolyzer, wherein the method includes passing at least one of
the water, oxygen and hydrogen through the fuel cell portion.
23. A method as claimed in claim 22, which includes, in the fuel
cell mode, passing water through the anode of the electrolyzer
portion, to maintain the electrolyzer portion heated.
24. A method as claimed in claim 23, which includes, in the fuel
cell mode, passing hydrogen for the fuel cell through the
electrolyzer cathode, providing at least one gas by-pass conduit in
the electrolyzer portion and passing oxidant for the fuel cell
through said at least one gas by-pass conduit, whereby the oxidant
and the hydrogen are preheated in the electrolyzer portion.
25. A method as claimed in claim 23, which includes, in the
electrolyzer mode of operation, passing water through a coolant
channel of the fuel cell portion and into the anode of the
electrolyzer, withdrawing oxidant and residual water from the anode
of the electrolyzer, and withdrawing hydrogen from the cathode of
the electrolyzer.
26. A method as claimed in claim 25, which includes withdrawing
hydrogen from the cathode of the electrolyzer through the anode of
the fuel cell portion.
27. A method as claimed in claim 26, which includes withdrawing
hydrogen through a conduit by-passing active areas of the fuel cell
portion.
28. A method as claimed in claim 27, which includes, in the
electrolyzer mode of operation, withdrawing hydrogen and entrained
water from a port located between the electrolyzer and fuel cell
portions and subsequently separating hydrogen from the water for
storage.
Description
FIELD OF THE INVENTION
This invention relates to a regenerative fuel cell system. More
particularly, this invention relates to a regenerative fuel cell
apparatus which combines a fuel cell unit and an electrolyzer unit,
and method of use thereof.
BACKGROUND OF THE INVENTION
Fuel cells have been proposed as a clean, efficient and
environmentally friendly power source that has various
applications. A conventional proton exchange membrane (PEM) fuel
cell is typically comprised of an anode, a cathode, and a selective
electrolytic membrane disposed between the two electrodes. A fuel
cell generates electricity by bringing a fuel gas (typically
hydrogen) and an oxidant gas (typically oxygen) respectively to the
anode and the cathode. In reaction, a fuel such as hydrogen is
oxidized at the anode to form cations (protons) and electrons by
the reaction H.sub.2 =2H.sup.+ +2e-. The proton exchange membrane
facilitates the migration of protons from the anode to the cathode
while preventing the electrons from passing through the membrane.
As a result, the electrons are forced to flow through an external
circuit thus providing an electrical current. At the cathode,
oxygen reacts with electrons returned from the electrical circuit
to form anions. The anions formed at the cathode react with the
protons that have crossed the membrane to form liquid water as the
reaction by-product following 1/2O.sub.2 +2H.sup.+ +2e-=H.sub.2 O.
On the other hand, an electrolyzer uses electricity to electrolyze
water to generate oxygen from its anode and hydrogen from its
cathode. Similar to a fuel cell, a typical solid polymer water
electrolyzer (SPWE) or proton exchange membrane (PEM) electrolyzer
is also comprised of an anode, a cathode and a proton exchange
membrane disposed between the two electrodes. Water is introduced
to, for example, the anode of the electrolyzer which is connected
to the positive pole of a suitable direct current voltage. Oxygen
is produced at the anode by the reaction H.sub.2 O=1/2O.sub.2
+2H.sup.+ +2e-. The protons then migrate from the anode to the
cathode through the membrane. On the cathode which is connected to
the negative pole of the direct current voltage, the protons
conducted through the membrane are reduced to hydrogen following
2H++2e-=H.sub.2.
It is well known in the art that one type of regenerative fuel cell
system combines separated fuel cell and electrolyzer units so that
during the fuel cell mode of the system, the fuel cell unit
generates electricity while consuming fuel gas (typically hydrogen)
and oxidant (typically oxygen or air) and during the electrolyzer
mode of the system, the electrolyzer unit generates the two process
gases for consumption by the fuel cell unit, i.e. oxygen and
hydrogen, while consuming electricity. Individual fuel cell and
electrolyzer cells are usually interconnected in a series
arrangement, often called "stacks".
U.S. Pat. No. 5,376,470 entitled "Regenerative Fuel Cell System"
and No. 5,506,066 entitled "Ultra-Passive Variable Pressure
Regenerative Fuel Cell System", both issued to Rockwell
International Corporation, disclose such a regenerative fuel cell
system. The regenerative fuel cell system comprises a fuel cell
including an anode for receiving hydrogen and a cathode for
receiving oxygen, an electrolyzer for electrolyzing water to
produce pure hydrogen and pure oxygen, storage tanks to
respectively store hydrogen and oxygen from the electrolyzer, a
water storage tank communicating with the fuel cell and the
electrolyzer. The fuel cell is located above the water storage tank
while the electrolyzer is located below the water storage tank.
Hydrogen is supplied to the fuel cell during fuel cell mode or
extracted from the cathode side of the electrolyzer during
electrolyzer mode via a hydrogen line that is connected to the
hydrogen storage tank and through a liquid-gas separator.
Similarly, oxygen is supplied to the fuel cell via lines and
through the water storage tank during fuel cell mode or extracted
from the anode side of the electrolyzer via an oxygen line and
through the water storage tank. The oxygen, when reaching the water
storage tank, bubbles up to the fuel cell via a supply line during
fuel cell mode or to the oxygen storage tank, when in the
electrolyzer mode.
However, these regenerative fuel cell systems cannot meet the
increasingly demanding requirement for fuel cell stacks. The
systems are usually large in size and heavy in weight and require
complex plumbing and ancillary equipment such as valves and
controls. As is known in the art, the performance of the fuel cell
unit in this system cannot be optimized unless an additional
humidification device is provided to humidify the process gases and
an additional heat exchanger is included to facilitate the heat
dissipation, all of which results in increased system size and
weight. When switching from electrolyzer mode to fuel cell mode,
the fuel cell unit in the conventional regenerative fuel cell
systems is cold and therefore is unable to achieve full power
output until the stack is warm.
Moreover, at present there is an expanding interest in vehicular
applications of fuel cell stacks, e.g. as the basic power source
for cars, buses and even larger vehicles. Vehicular applications
are quite different from many stationary applications. In
stationary applications, fuel cell stacks are usually used as an
electrical power source and are simply expected to run at a
relatively constant power level for an extended period of time. In
contrast, in a vehicular, particularly an automotive environment,
the actual power required from the fuel cell stack can vary
significantly. Moreover, the fuel cell stack is expected to respond
rapidly to changes in power demand while maintaining high
efficiencies. Further, for vehicular, particularly automotive
applications, a fuel cell power unit is expected to operate under a
disparate range of ambient temperature and humidity conditions. In
addition, during regenerative braking period, the prior
regenerative fuel cell systems are unable to capture the
electricity to recharge the system due to their slow switchover
times, making them less efficient. All these requirements are
exceedingly demanding and make it difficult to incorporate a
conventional regenerative fuel cell system into a vehicle and
operate efficiently.
In view of the disadvantages and drawbacks associated with
conventional regenerative fuel cell systems, it is desirable to
provide a regenerative fuel cell system that enables improved fuel
cell performance, including rapid switchover between fuel cell and
electrolyzer modes, instantaneous full power operation, higher
power density, less peripherals and hence higher system
efficiency.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a
regenerative fuel cell system is provided, comprising an
electrolyzer portion and a fuel cell portion;
the electrolyzer portion has a closeable hydrogen inlet and a
hydrogen outlet in communication with the cathode of the
electrolyzer portion for conducting hydrogen, a gas bypass having a
gas bypass inlet and a gas bypass outlet for conducting oxidant gas
for fuel cell reaction to the fuel cell portion, a water inlet and
an oxygen-water outlet for exhausting oxygen generated in
electrolyzer operation and coolant water from the fuel cell portion
out of the electrolyzer portion;
the fuel cell portion has a hydrogen inlet, a first closeable
hydrogen outlet for exhausting excess hydrogen in fuel cell mode, a
second closeable hydrogen outlet for exhausting hydrogen generated
in the electrolyzer portion in electrolyzer mode, an oxidant gas
inlet, an oxidant gas outlet, a coolant water inlet and a coolant
water outlet; and
the hydrogen inlet of the fuel cell portion being in communication
with the hydrogen outlet of the electrolyzer portion; the oxidant
gas inlet of the fuel cell portion being in communication with the
gas bypass outlet of the electrolyzer portion; and the water inlet
of the electrolyzer portion being in communication with the coolant
water outlet of the fuel cell portion.
Preferably, the fuel cell portion and the electrolyzer portion are
in juxtaposition. More preferably, the fuel cell portion and the
electrolyzer portion are stacked one on top of the other, with the
electrolyzer portion on the top.
The fuel cell portion comprises at least one proton exchange
membrane fuel cell having an anode bipolar plate and a cathode
bipolar plate; and the electrolyzer portion comprises at least one
proton exchange membrane electrolyzer cell having an anode bipolar
plate and a cathode bipolar plate.
Preferably, the regenerative fuel cell system further includes a
separator plate sandwiched between the fuel cell portion and the
electrolyzer portion, said separator plate is provided with a
hydrogen port functioning as the hydrogen inlet for the fuel cell
portion and the hydrogen outlet for the electrolyzer portion, an
oxidant gas port functioning as the oxidant gas inlet for the fuel
cell portion and the gas bypass outlet for the electrolyzer
portion, and a water port functioning as the coolant water outlet
for the fuel cell portion and the water inlet for the electrolyzer
portion.
More preferably, the said second closeable hydrogen outlet is in
alignment with the said hydrogen inlet of the fuel cell portion.
More preferably, a common current collector plate is sandwiched
between the anode of the fuel cell portion and the cathode of the
electrolyzer portion, and said current collector plate is provided
with a hydrogen port functioning as the hydrogen inlet for the fuel
cell portion and the hydrogen outlet for the electrolyzer portion,
an oxidant gas port functioning as the oxidant gas inlet for the
fuel cell portion and the gas bypass outlet for the electrolyzer
portion, and a water port functioning as the coolant water outlet
for the fuel cell portion and the water inlet for the electrolyzer
portion. Alternatively, the common current collector plate is
grounded.
More preferably, the said gas bypass of the electrolyzer portion is
provided on the face of the anode bipolar plate of each
electrolyzer cell facing away from the proton exchange
membrane.
More preferably, the said separator plate further includes a switch
means in fluid communication with the hydrogen outlet of the
electrolyzer portion, the hydrogen inlet of the fuel cell portion
and an external hydrogen storage means, and said switch means
operatively switches between a first position in which it fluidly
communicates the hydrogen outlet of the electrolyzer portion to the
external hydrogen storage means, and a second position in which it
fluidly communicates the hydrogen outlet of the electrolyzer
portion with the hydrogen inlet of the fuel cell portion.
More preferably, the electrolyzer portion further includes an
additional hydrogen outlet for supplying the hydrogen generated in
electrolyzer mode to an external storage means, and a purge means
that purges the water carried by the hydrogen generated in the
electrolyzer mode.
According to a second aspect of the present invention, a method of
operating the regenerative fuel cell system in the first aspect of
the present invention is provided, wherein comprising:
in fuel cell mode, introducing hydrogen into the electrolyzer
portion via the said closeable hydrogen inlet so that hydrogen
flows across the cathode of the electrolyzer portion before
entering the fuel cell portion for reaction; introducing oxidant
gas into the electrolyzer portion via the said gas bypass inlet so
that the oxidant gas flows along the gas bypass and leaves the
electrolyzer portion before entering the fuel cell portion;
introducing coolant water into the electrolyzer portion after the
coolant water flows through the fuel cell portion so that the
coolant water flows across the anode of the electrolyzer portion;
and
in electrolyzer mode, closing the said closeable hydrogen inlet and
running the hydrogen generated into the fuel cell portion through
the fuel cell portion; introducing coolant water of the fuel cell
portion into the anode of the electrolyzer portion after the
coolant water flows through the fuel cell portion.
The structure of the regenerative fuel cell system according to the
present invention provides significant advantages over the existing
system. First of all, the switchover time between the two operation
modes is reduced to a minimum because the exchange of water and gas
streams between the electrolyzer and the fuel cell portions ensures
the reactant gases and liquid are on the proper electrodes for each
reaction. In addition, both the fuel cell portion and electrolyzer
portion are able to achieve full power instantaneously after the
system is switched from one mode to the other due to the exchange
of water and gas streams between the two sections. This water and
gas exchange maintains both the fuel cell and electrolyzer portions
of the stack at full operating temperature as well as maintaining
the fuel cell portion in a humidified condition. In fact, the
electrolyzer portion functions as a humidification section for the
fuel gases, i.e. hydrogen for the fuel cell portion so that the
higher temperature of operation is possible without drying out the
MEA of the fuel cells. The electrolyzer portion also functions as a
heat exchanger for the fuel cell portion to dissipate heat as a
result of the exchange of water and gases between the two portions.
Further, the electrolyzer portion preheats the fuel cell supply
gases, preventing condensation and flooding in the first cells of
the fuel cell portion, a common problem with cold gas streams. This
heat exchange process serves to warm up the electrolyzer portion
itself, improving the performance of the electrolyzer when
switching to the electrolyzer mode of operation. In addition, the
present system even allows for simultaneous fuel cell and
electrolyzer operation, eliminating switchover time between the two
modes of operation. This further demonstrates the instant on
capability of the regenerative fuel cell system, making it suitable
for deployment in vehicular applications for the reasons outlined
in the aforementioned background technology. The rapid switchover
time enables the system to capture the electricity energy to
recharge the system during regenerative braking period when it is
applied in vehicular applications, thereby making the regenerative
fuel cell system more efficient. This rapid switchover time also
makes the system well suited to UPS power type applications where
seamless transfer between power generation and power storage modes
of operation are required.
Further, since the electrolyzer and fuel cell portions share the
single water cooling and humidification loop, and since most of the
cooling and humidification happens internally, the system requires
less plumbing and less pipe or conduit components. Therefore the
structure of the system is simplified, resulting in reduced size
and weight.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, and to show
more clearly how it may be carried into effect, reference will now
be made to the accompanying drawings which show, by way of example,
preferred embodiments of the present invention, and in which;
FIG. 1 is a schematic view, which illustrates a conventional
regenerative fuel cell system;
FIGS. 2a and 2b are exploded perspective views, which illustrate
the regenerative fuel cell system according to a first embodiment
of the present invention;
FIG. 3a is a schematic view, which illustrates the hydrogen flow in
the regenerative fuel cell system according to the first embodiment
of the present invention when the system runs in fuel cell
mode;
FIG. 3b is a schematic view, which illustrates the oxygen or air
flow in the regenerative fuel cell system according to the first
embodiment of the present invention when the system runs in fuel
cell mode;
FIG. 3c is a schematic view, which illustrates the water flow in
the regenerative fuel cell system according to the first embodiment
of the present invention when the system runs in fuel cell
mode;
FIG. 4a is a schematic view, which illustrates the water flow in
the regenerative fuel cell system according to the first embodiment
of the present invention when the system runs in electrolyzer
mode;
FIG. 4b is a schematic view, which illustrates the hydrogen flow in
the regenerative fuel cell system according to the first embodiment
of the present invention when the system runs in electrolyzer
mode;
FIG. 4c is a schematic view, which illustrates the air oxygen flow
in the regenerative fuel cell system according to the first
embodiment of the present invention when the system runs in
electrolyzer mode;
FIGS. 5a and 5b are a schematic views, which illustrate the flows
of the process gases and water in both the electrolyzer and fuel
cell portions of the regenerative fuel cell system according to the
present invention in the fuel cell mode and the electrolyzer modes
respectively;
FIG. 6 is an exploded rear front perspective view, respectively,
which illustrate a second embodiment of the regenerative fuel cell
system according to the present invention;
FIG. 7 is an exploded front perspective view, which illustrates a
third embodiment of the regenerative fuel cell system according to
the present invention;
FIG. 8a is a schematic view, which illustrates a separator plate
incorporating a switch means of the regenerative fuel cell system
according to the present invention, and FIG. 8b is an expanded view
of part of FIG. 8a; and
FIG. 9 is a schematic view, which illustrates a separator plate
incorporating a purge valve of the regenerative fuel cell system
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A conventional regenerative fuel cell system, for example, as
disclosed in U.S. Pat. No. 5,376,470 is shown in FIG. 1. The
regenerative fuel cell system comprises a fuel cell 112 including
an anode for receiving hydrogen and a cathode for receiving oxygen,
an electrolyzer 116 for electrolyzing water to produce pure
hydrogen and pure oxygen, storage tanks 124, 126 to respectively
store hydrogen and oxygen, and water storage tank 114 communicating
with said fuel cell 112 and the electrolyzer 116. The fuel cell 112
is located above the water storage tank 114 while the electrolyzer
116 is located below the water storage tank 114. Hydrogen is
supplied to the fuel cell 112 during fuel cell mode or extracted
from the cathode side of the electrolyzer 116 during electrolyzer
mode via a hydrogen line 128 that is connected to the hydrogen
storage tank 124 and through a liquid-gas separator 122. A flow
valve 120 and a secondary water storage tank 118 are provided, for
humidifying the hydrogen stream. Similarly, oxygen is supplied to
the fuel cell 112 via lines 130 and 115 and through the water
storage tank 114 during fuel cell mode or extracted from the anode
side of the electrolyzer 116 via oxygen line 130 and 117 and
through the water storage tank 114. In the electrolyzer mode, the
oxygen generated flows up to the water storage tank 114 and then
bubbles up to the fuel cell via line 115 if the fuel cell is in the
fuel cell mode or to oxygen storage means 126 via line 130 if the
fuel cell is not operating.
FIG. 2a shows a first embodiment of the regenerative fuel cell
system according to the present invention. In the following
description, for the purpose of illustration, fuel cells are
described by taking PEM fuel cells as an example, and likewise, the
process gases discussed hereinafter are limited to pure hydrogen
and air. However, it should be appreciated that the fuel cell in
the present invention is not limited to PEM fuel cells and the
process gases can be different; for example it is common to use air
as a source of an oxidant gas.
The present regenerative fuel cell system 10 includes a fuel cell
portion 20 and an electrolyzer portion 30. In practice, each of the
fuel cell portion 20 and the electrolyzer portion 30 may consist of
a plurality of individual cells that are stacked in series. For
simplicity, in FIG. 2a the fuel cell portion 20 is shown including
only one fuel cell and similarly the electrolyzer portion 30 is
shown including only one electrolyzer cell.
Each fuel cell comprises an anode bipolar plate 21, a cathode
bipolar plate 22 and a membrane electrode assembly (MEA) 23
sandwiched between the anode 21 and cathode 22. The MEA 23 has a
proton exchange membrane (PEM) 24. It is to be understood that
designations "front" "rear" used hereinafter with respect to the
anode and cathode bipolar plates of fuel cells and electrolyzer
cells, indicates their orientation with respect to the MEA 23.
Thus, "front" indicates the face towards the MEA 23; the indication
"rear" indicates the face away from the MEA 23. Each of the anode
bipolar plate 21 and the cathode bipolar plate 22 is provided with
a flow field 25 on the front face thereof, for respectively
introducing a process gas, e.g. hydrogen or oxygen to one side of
the said membrane 24 for reaction. The anode and cathode bipolar
plates also have coolant flow fields on the respective back faces
thereof, which allow the coolant to pass through chambers between
and adjacent the fuel cells, to remove heat from the fuel cell
since the reaction is exothermic.
Three inlets are provided on one end of each bipolar plate 21, 22
respectively for hydrogen, air or oxygen, and a coolant, usually
water. On the opposite end of each bipolar plate 21, 22, three
corresponding outlets are provided. These inlets and outlets, in
known manner, include openings extending through the bipolar plates
21, 22, so that, in a complete fuel cell stack with many individual
fuel cells, the ports align to form continuous supply ducts for the
various fluids.
A gas diffusion media (GDM) 26 is interposed between one surface of
the MEA 23 and the anode bipolar plate 21, and similarly another
GDM 26 is placed between the opposite surface of the MEA 23 and the
cathode bipolar plate 22, to enable the diffusion of the process
gas, either the fuel or oxidant, to the surface of the MEA 23 and
provide conduction of electricity between the respective bipolar
plates 21, 22 and the MEA 23. Two current collector plates 27
respectively abut against rear surfaces of the outermost bipolar
plates 21, 22 of the fuel cell stack, i.e. opposite to the surface
adjacent to the MEA 23, to collect the current from the bipolar
plates 21, 22 and connect to an external circuit (not shown). To
complete the whole stack, two insulator plates 28 are provided
abutting against the outer surfaces of the current collector plates
27 and two end plates 40, 80 are also provided abutting against the
outer surfaces of the insulator plates 28. The end plates 40, 80
are provided for structural reasons and can be used to press the
electrolyzer stack together by means of bolts, etc.
Similarly, the electrolyzer portion 30 is shown to include only one
electrolyzer cell. In this embodiment, a conventional structure of
electrolyzer cells is described by way of example, and in many ways
this corresponds to the structure of the fuel cells. Each
electrolyzer cell comprises an anode bipolar plate 31, a cathode
bipolar plate 32 and a membrane electrode assembly (MEA) 33
sandwiched between the anode 31 and cathode 32. The MEA 33 has a
proton exchange membrane (PEM) 34. The anode bipolar plate 31 is
provided with a flow field 35 on the front face thereof for
introducing water to one side of the membrane 34 for reaction. On
the rear face of the anode bipolar plate 31, an air bypass flow
field (not shown) is provided forth air used in fuel cell reaction
to bypass the electrolyzer portion. The cathode bipolar late 32 is
also provided with a flow field 35 on the front surface thereof for
conducting the hydrogen through the electrolyzer portion 30 to the
fuel cell portion 20 (either enerated in the electrolyzer reaction
or to be used in the fuel cell reaction from external storage
means). It is to be understood that in an electrolyzer stack
comprising a plurality of electrolyzer cells, the air bypass flow
field is formed by the rear face of the anode late of one cell and
the rear face of the cathode plate of an adjacent cell abutting
against this anode plate. Either the rear face of the anode plate
or the rear face of the cathode plate (or both) may be provided
with flow channel to form the flow field while the other face is
smooth.
Three inlets are provided adjacent one end of each bipolar plate
31, 32 respectively for hydrogen, air or oxygen and coolant,
usually water. Three corresponding outlets are provided adjacent
the opposite end of each bipolar plate 31, 32. As for the fuel cell
portion 20, the inlets and outlets are openings extending through
the bipolar plates 31, 32. A gas diffusion media (GDM) 36 is
interposed between one surface of the MEA 33 and the anode bipolar
plate 31, and similarly another GDM 36 is placed between the
opposite surface of the MEA 33 and the cathode bipolar plate 32. On
the anode, the gas diffusion media enables the diffusion of the
process water to the surface of the MEA 33 and provides conduction
of electricity between the bipolar plate 31 and the MEA 33. On the
cathode, the gas diffusion media enables the diffusion of the
product gas (hydrogen) away from the surface of the MEA 33 and
provides conduction of electricity between the bipolar plate 32 and
the MEA 33.
Two current collector plates 37 respectively abut against rear
surfaces of the outermost bipolar plates 31, 32, i.e. opposite to
the surfaces adjacent to the MEA 33, to collect the current from
the bipolar plates 31, 32 and connect to an external circuit (not
shown). To complete the whole stack, two insulator plates 38 are
provided abutting against the outer surfaces of the current
collector plates 37 and two end plates 40, 70 are also provided
abutting against the outer surfaces of the insulator plates 38.
Pressure again can be applied on the end plates 40, 70 to press the
stack together by means of bolts, etc.
In the present invention, the fuel cell portion 20 and he
electrolyzer portion 30 are preferably stacked together. Therefore,
only one end late is needed between the two portions, which, as
noted above, is the common separator plate 40. As can be seen in
FIG. 2b, the end plate 80 of the fuel cell portion 20 has four
ports each communicating between the internal flow fields of the
fuel cell portion 20 and outside pipelines or conduits to allow
process gases and coolant to flow through, specifically, a fuel
cell hydrogen outlet 50, an electrolyzer hydrogen out 51, a fuel
cell air outlet 52 and a water inlet 53. The end plate 70 of the
electrolyze portion 30 has three ports communicating between the
internal flow fields of the electrolyzer portion 30 and outside
pipelines, specifically, a fuel cell hydrogen inlet 60, an air
inlet 62 and a water outlet 63. Similarly, on the separator plate
40, there are provided three ports corresponding to the three ports
of the electrolyzer end plate 70 to communicate between the fuel
cell and electrolyzer portions on opposite sides of the separator
plate 40, specifically a hydrogen port 41, an air port 42 and a
water port 43. On the end plates 70, 80, the ports are adapted to
connect to pipes or conduits as well as valves or switches. On the
separator plate 40, the ports are preferably in the norm of through
holes with necessary seals provided around them. It is to be
understood hat the relative position sequence of the anode and
cathode bipolar plates in either fuel cell portion or electrolyzer
portion may be different and this will not affect the operation of
the regenerative fuel cell system of the present invention. In
other words, the bipolar plate immediately adjacent the separator
plate 40 in the fuel cell portion 20 ay be either an anode bipolar
plate or a cathode bipolar plate. Likewise, the bipolar plate
immediately adjacent the separator plate 40 in the electrolyzer
portion 30 may be either an anode bipolar plate or a cathode
bipolar plate.
The various ports for fuel cell and electrolyzer portions 20, 30
have been described above as being, an "inlet" and "outlet".
However, it is to be appreciated that in general flows of the
various fluids may be reversed as between the fuel cell and
electrolyzer modes; alternatively, or as well, for some modes of
operation, while flow directions for a fluid may be the same in
both electrolyzer and fuel cell modes, the flow direction could be
different from that described. Accordingly, the ports can be
identified, more generically as follows. First, on the end plate 70
for the electrolyzer portion 30, the ports 60, 62 and 63 can be
considered as a first electrolyzer cathode port 60, a first gas
bypass port 62 and a first electrolyzer anode port 63. The port 62
is designated as a "bypass port", since the oxidant is considered
to "bypass" the active area of the electrolyzer cells. For the end
plate 80, the ports 50, 51, 52 and 53 can be considered as: a first
fuel cell anode port 50; third fuel cell anode port 51; a first
fuel cell cathode port 52; and a first coolant port 53; the port 51
is designated as a third port, as it is, in some senses, optional
depending upon the configuration for extracting hydrogen during
electrolysis, and the designation of the second port is reserved
for the hydrogen or anode port at the other end of the fuel cell
portion.
The middle or separator plate 40 then has the three parts 41, 42
and 43, which provide second ports for both the fuel cell portion
and the electrolyzer portion, as follows: port 41 provides a second
cathode port of the electrolyze and a second anode port of the fuel
cell portion; port 42 provides a second air or gas bypass port of
the electrolyzer portion and a second cathode port of the fuel cell
portion; port 43 provides a second anode port of the electrolyzer
portion and a second coolant port of the fuel cell portion.
In the following description of the operation of the fuel cell
portion 20 and the electrolyzer portion 30, the previous
designations "inlet" and "outlet" are used. However, it will be
appreciated that, as mentioned, in some applications, it may be
desirable to reverse the flows. Reversing flows will have
implications for heat transfer as between the fuel cell and the
electrolyzer portions 20, 30 and this will need to be taken into
account in determining desirable flow directions.
Now the operation in two modes of the regenerative fuel cell system
according to the present invention will be described in detail with
reference being made to FIGS. 3a-3c, 4a-4c and 5a-5b.
Fuel Cell Operation
(1) Hydrogen Flow
FIG. 3a illustrates the hydrogen flow when the regenerative fuel
cell system runs in the fuel cell mode. As can be seen in FIG. 3a,
the hydrogen first enters the electrolyzer portion 30 of the
regenerative fuel cell stack from a hydrogen storage means (not
shown) via the fuel cell hydrogen inlet 60 provided on the
electrolyzer end plate 70 and flows to the hydrogen inlet of the
flow field 35 adjacent one end of cathode bipolar plate 32 of each
electrolyzer cell. The hydrogen flows from the hydrogen inlet,
crosses the cathode flow field 35 of each electrolyzer cell (FIG.
5a) and reaches the hydrogen outlet of the flow field 35 adjacent
the opposite end of the cathode bipolar plate 32 of each
electrolyzer cell. Then the hydrogen exits the flow fields 35 of
the electrolyzer cells via the hydrogen outlets and leaves the
electrolyzer portion 30, without any reaction, via the hydrogen
port 41 provided on the separator plate 40. In this embodiment, as
is known in the art, the fuel cell hydrogen port 60 on the
electrolyzer end plate 70 is in alignment with the hydrogen inlets
of flow fields 35 on the cathode bipolar plates 32 of the
electrolyzer cells. Likewise, the hydrogen port 41 on the separator
plate 40 is in alignment with the hydrogen outlets of the flow
fields 35 of the cathode bipolar plates 32 of the electrolyzer
cells.
After passing through the hydrogen port 41, the hydrogen enters the
fuel cell portion 20 and flows to the hydrogen inlet of the flow
field 25 adjacent one end of the anode bipolar plate 21 of each
fuel cell. The hydrogen then enters the flow fields 25 of the anode
bipolar plates 21 from the hydrogen inlets thereof. The hydrogen
spreads across the flow field 25 and, in known manner, reacts on
the anode side of the proton exchange membranes 24 of each fuel
cell in the presence of a catalyst, generating protons that pass
through the membrane (FIG. 5b). The unreacted hydrogen continues to
flow and exits the flow field 25 of each fuel cell via the hydrogen
outlet of the flow field 25 adjacent the opposite end of the anode
bipolar plate 21 of each fuel cell. The hydrogen then leaves the
fuel cell portion 20 via the fuel cell hydrogen outlet 50 provided
on the fuel cell end plate 80 and returns to the hydrogen storage
means or is exhausted to the environment. In this embodiment, as is
known in the art, the hydrogen port 41 on the separator plate 40 is
in alignment with the hydrogen inlets of the flow fields 25 of the
anode bipolar plates 21 of the fuel cells. Likewise, the fuel cell
hydrogen outlet 50 on the fuel cell end plate 80 is in alignment
with the hydrogen outlets of the flow fields 25 of the anode
bipolar plates 21 of the fuel cells.
It should be mentioned that on the fuel cell end plate 80, another
hydrogen outlet is provided, i.e. the electrolyzer hydrogen outlet
51, which is in alignment with the hydrogen inlets of the flow
fields 25 of the anode bipolar plates 21 of the fuel cells.
However, when the regenerative fuel cell system runs in the fuel
cell mode, the fuel cell hydrogen outlet 50 on the fuel cell end
plate 80 is in an open position while the electrolyzer hydrogen
outlet 51 is in closed position. Therefore, after entering the fuel
cell portion 20 via the hydrogen port 41, the hydrogen cannot exit
the fuel cell portion 20 by directly flowing to the electrolyzer
hydrogen outlet 51. On the contrary, the hydrogen is forced to flow
across the flow fields 25 and out of the fuel cell hydrogen outlet
50, which is then the only passage available.
(2) Air Flow
FIG. 3b illustrates the air flow when the regenerative fuel cell
system runs in the fuel cell mode. As can be seen in FIG. 3b,
ambient air, or other suitable oxidant, first enters the
electrolyzer portion 30 from an air supply means, usually a
compressor, a blower or a fan (not shown), via the fuel cell air
inlet 62 on the electrolyzer end plate 70 and flows to an air inlet
port of the air bypass flow field of the anode bipolar plate 31 of
each electrolyzer cell, provided adjacent one nd on the rear side
of each anode bipolar plate 31. The air flows from the air inlets,
crosses the air bypass flow fields of the electrolyzer cell and
reaches the air outlets of the air bypass flow fields, adjacent the
opposite ends of the anode bipolar plates 31, on the rear side of
the anode bipolar plate 31 of each electrolyzer cell (FIG. 5a). In
practice, when the regenerative fuel cell system switches from
electrolyzer mode to fuel cell mode, passing the air through the
relatively hot electrolyzer cells will be sufficient to heat the
air up to the fuel cell operating temperature which is critical for
proper fuel cell peration; once the fuel cell has been operating
for some time, it will be sufficiently hot that upstream
pre-heating of the air is not essential. Then the air exits the air
bypass flow field of each electrolyzer cell via the air outlet and
leaves the electrolyzer portion 30 without any reaction via the air
port 42 provided on the separator plate 40. In this embodiment, as
is known in the art, the fuel cell air in let 62 on the end plate
70 is in alignment with the air inlets of the air bypass flow
fields on the rear sides of the anode bipolar piates 31. Likewise,
the air port 42 on the separator plate 40 is in alignment with the
air outlets of the air bypass flow fields on the rear sides of
anode bipolar plates 31 the air bypass flow fields collectively
providing an air bypass conduit.
After passing through the air port 42, the air enters the fuel cell
portion 20 and flows to the inlet of the flow field 25 adjacent one
end of the cathode bipolar plate 22 of each fuel cell. The air then
enters the flow fields 25 of the cathode bipolar plates 22 from the
inlets thereof. The air spreads across the flow fields 25 and
reacts on the cathode side of the proton exchange membranes 24 of
each fuel cell in the presence of catalyst (FIG. 5b). The unreacted
air continues to flow and exits the flow field 25 of each fuel cell
via the outlet of the flow field 25 adjacent the opposite end of
the cathode bipolar plate 22 of each fuel cell. The air then leaves
the fuel cell portion 20 via the fuel cell air outlet 52 provided
on the fuel cell end plate 80. In this embodiment, as is known in
the art, the air port 42 on the separator plate 40 is in alignment
with the air inlets of the flow fields 25 on the cathode bipolar
plates 22 of the fuel cells. Likewise, the fuel cell air outlet 52
on the fuel cell end plate 80 is in alignment with the air outlets
of the flow fields 25 of the cathode bipolar plates 22 of the fuel
cells.
(3) Water Flow
Water works as a conventional coolant during fuel cell operation as
the fuel cell reaction is an exothermic reaction. FIG. 3c
illustrate water flow when the regenerative fuel cell system runs
in the fuel cell mode. As can be seen in FIG. 3c, the water first
enters the fuel cell portion 20 from a water storage means (not
shown) via the water inlet 53 provided on the fuel cell end plate
80 and flows to the respective inlet of the coolant flow field
adjacent one end of the anode and cathode bipolar plates 21, 22 of
each fuel cell. In PEM fuel cell applications, the coolant flow
fields are usually provided on the respective rear faces of the
bipolar plates 21, 22. Water flows from the water inlets, crosses
the coolant flow fields of the fuel cell and reaches the water
outlets of the coolant flow fields adjacent the opposite end of the
anode and cathode bipolar plates 21, 22 of each fuel cell (FIG.
5b). Then the water exits the coolant flow fields of the fuel cell
via the outlets and leaves the fuel cell portion 20 via the water
port 43 provided on the separator plate 40. In this embodiment, as
is known in the art, the water inlet 53 on the fuel cell end plate
80 is in alignment with the inlets of the coolant flow fields on
the anode and cathode bipolar plates 21, 22 of the fuel cells.
Likewise, the water port 43 on the separator plate 40 is in
alignment with the outlets of the coolant flow fields of the anode
and cathode bipolar plates 21, 22 of the fuel cells.
After passing through the water port 43, the water enters the
electrolyzer portion 30 and flows to each water inlet of the flow
field 35 adjacent one end of the anode bipolar plate 31 of each
electrolyzer cell. The water then enters the flow fields of the
anode bipolar plates 31 from the water inlets thereof. The water
spreads across the flow fields 35 (FIG. 5a), and exits the flow
field 35 of each electrolyzer cell via the water outlet of the flow
field 35 adjacent the opposite end of the anode bipolar plate 31 of
each electrolyzer cell. The water then leaves the electrolyzer
portion 30 via the water outlet 63 provided on the electrolyzer end
plate 70 and goes to a water storage means (not shown) from which
it is supplied to the electrolyzer cells as a reactant during
electrolyzer operation. In this embodiment, as is known in the art,
the water port 43 on the separator plate 40 is in alignment with
the water inlets of the flow fields 35 on the anode bipolar plates
31 of the electrolyzer cells. Likewise, the water outlet 63 on the
electrolyzer end plate 70 is in alignment with the water outlets of
the flow fields 35 of the anode plates 31 of electrolyzer
cells.
As can be appreciated from FIGS. 3a to 3c, during fuel cell
operation, water continuously flows across the anode of the
electrolyzer cells and when the system switches from fuel cell
operation to electrolyzer operation, water is present at the anode
electrode of each electrolyzer cell. The presence of water also
prevents the electrolyzer membrane from drying out. In addition,
water from the fuel cell portion is heated by the fuel cell
reaction and in turn warms up the electrolyzer portion. Hence the
electrolyzer portion is capable of operating at optimum conditions
immediately and the switching time between the two modes is reduced
to a minimum. Therefore, the performance of electrolyzer mode of
the present system is significantly improved, particularly at
startup. It should be appreciated that in practice, the coolant
flow field may be provided only on the back face of the anode
bipolar plate 21 or the back face of the cathode bipolar plate 22
of each fuel cell. However, for illustration purpose, both the
anode and cathode bipolar plates 21, 22 are provided with coolant
flow field in this embodiment.
Also, the membrane of the electrolyzer cells can be such as to
enable water to diffuse through it into the hydrogen flow. Thus,
the electrolyzer portion 30 then acts as a humidifier, humidifying
incoming hydrogen. Heating the water in the fuel cell portion 20
further promotes this humidification process.
Electrolyzer Operation
(1) Water Flow
Water is the reactant during the electrolysis reaction. FIG. 4a
illustrates the water flow when the regenerative fuel cell system
runs in the electrolyzer mode. As can be seen in FIG. 4a, water
first enters the fuel cell portion 20 from a water storage means
(not shown) via the water inlet 53 provided on the fuel cell end
plate 80 and flows to the respective inlet of the coolant flow
field adjacent one end of the anode and cathode bipolar plates 21,
22 of each fuel cell. As explained above, in PEM fuel cell
applications, the coolant flow fields are usually provided on the
respective rear faces of the bipolar plates 21, 22. Water flows
from the water inlets, crosses the coolant flow fields of the fuel
cells and reaches the outlets of the coolant flow fields adjacent
the opposite end of the anode and cathode bipolar plates 21, 22 of
each fuel cell (FIG. 5b). Then water exits the coolant flow fields
of the fuel cell via the water outlets and leaves the fuel cell
portion 20 via the water port 43 provided on the separator plate
40. In this embodiment, as is known in the art, the water inlet 53
on the fuel cell end plate 80 is in alignment with the water inlets
of the coolant flow fields on the anode and cathode bipolar plates
21, 22 of the fuel cells. Likewise, the water port 43 on the
separator plate 40 is in alignment with the water outlets of the
coolant flow fields of the anode and cathode bipolar plates 21, 22
of the fuel cells. In fact, the water flow within the fuel cell
portion 20 in the electrolyzer mode is identical to that in the
fuel cell mode, as shown in FIG. 3c.
After passing through the water port 43, water enters the
electrolyzer portion 30 and flows to the water inlet of the anode
flow field 35 adjacent one end of the anode bipolar plate 31 of
each electrolyzer cell. The water then enters the flow fields 35 of
the anode bipolar plates 31 from the water inlets thereof from the
said water storage means. The water spreads across the flow fields
35 and is electrolyzed, generating hydrogen and oxygen at the
cathodes and anodes respectively (FIG. 5a). The unreacted water
combined with the generated oxygen exits the anode flow field 35 of
each electrolyzer cell via the water outlet of the flow field 35
adjacent the opposite end of the anode bipolar plate 31 of each
electrolyzer cell. The water and generated oxygen then leave the
electrolyzer portion 30 via the water outlet 63 provided on the
electrolyzer end plate 70, and the water returns to the said water
storage means. In this embodiment, as is known in the art, the
water port 43 on the separator plate 40 is in alignment with the
water inlets of the flow fields 35 on the anode bipolar plates 31
of the electrolyzer cells. Likewise, the water outlet 63 on the
electrolyzer end plate 70 is in alignment with the water outlets of
the flow fields 35 of the anode bipolar plates 31 of the
electrolyzer cells. Again, the water flow within the electrolyzer
portion 20 in the electrolyzer mode is substantially similar to
that in the fuel cell mode of FIG. 3c. The difference is that water
acts as a reactant in the electrolyzer mode.
(2) Hydrogen Flow
FIG. 4b illustrates the hydrogen flow when the regenerative fuel
cell system runs in the electrolyzer mode. As is known in the art,
hydrogen is the product of the electrolysis reaction on the cathode
of the electrolyzer cells. During electrolyzer operation, the
hydrogen inlet 60 provided on the electrolyzer end plate 70 is in
closed position so that when hydrogen is generated within the
electrolyzer portion 30, it will not flow across the cathode flow
field 35 of each electrolyzer cell and exit the cathode flow field
35 via the inlet thereof adjacent one end of the cathode bipolar
plate 32 of each electrolyzer cell. Instead, hydrogen flows across
the cathode flow field 35 of each electrolyzer cell (FIG. 5a) and
reaches the hydrogen outlet of the flow field 35 adjacent the
opposite end of each cathode bipolar plate 32 since it is the only
passage available. Then the hydrogen exits the cathode flow fields
35 of the electrolyzer cells via the hydrogen outlets and leaves
the electrolyzer portion 30 via the hydrogen port 41 provided on
the separator plate 40. As is known in the art, the hydrogen port
41 on the separator plate 40 is in alignment with the hydrogen
outlets of the flow fields 35 of the cathode bipolar plates 32 of
the electrolyzer cell.
After passing through the hydrogen port 41, the hydrogen enters the
fuel cell portion 20 and flows to the hydrogen inlet of the flow
field 25 adjacent one end of the anode bipolar plate 21 of each
fuel cell. It should be mentioned that during electrolyzer
operation, the fuel cell hydrogen outlet 50 provided on the fuel
cell end plate 80 is in closed position while the electrolyzer
hydrogen outlet 51 is in an open position. Consequently, when
hydrogen enters the flow fields 25 of the anode bipolar plates 21
from the hydrogen inlets thereof and spreads across the flow fields
25 (FIG. 5b), it will not flow via the outlets of the flow fields
25 adjacent the opposite end of anode bipolar plates 21 to the fuel
cell hydrogen outlet 50. Although hydrogen may also flow across the
flow field 25 to the said outlet, it is forced to flow along the
channel formed by the inlets of the anode flow fields 25 to the
electrolyzer hydrogen outlet 51 provided on the fuel cell end plate
80, which is the only passage available. The hydrogen then leaves
the fuel cell portion 20 via the electrolyzer hydrogen outlet 51
and flows to the hydrogen storage means from which it is supplied
to the fuel cells as a reactant during fuel cell operation. In this
embodiment, as is known in the art, both the hydrogen port 41 on
the separator plate 40 and the electrolyzer hydrogen outlet 51 on
the fuel cell end plate 80 are in alignment with the hydrogen
inlets of the flow fields 25 on the anode bipolar plates 21 of the
fuel cells.
As is known in the art, hydrogen generated during electrolyzer
operation carries water with it as it flows out of the electrolyzer
portion. This water may flood fuel cells when it comes into the
fuel cell portion and continuously flows across fuel cell anodes
together with the hydrogen. However, in the present invention,
almost all the hydrogen generated in electrolyzer operation flows
directly to the electrolyzer hydrogen outlet 51 through the channel
formed by the inlets of the anode flow fields 25. Although some
hydrogen may also flow across the flow field 25 to fuel cell
hydrogen outlet 50, since the flow passage is closed, the actual
situation is some hydrogen is stagnant on the anode flow field 25
of the fuel cell portion. Therefore, flooding problem is easily
avoided. Moreover, in case that the system switches from fuel cell
mode to electrolyzer mode, the unconsumed hydrogen present in the
anode flow filed 25 in the fuel cell portion 20 also helps to
prevent the fuel cell portion 20 from being flooded.
When the regenerative fuel cell system is switching from the
electrolyzer mode to the fuel cell mode, the presence of hydrogen
in the fuel cell portion 20 from the generation of hydrogen in the
electrolyzer portion 30 ensures an instantaneously available source
of fuel for the fuel cell reaction and no purge period is needed.
The heat generated in the electrolyzer reaction also keeps the
stack in an elevated temperature. Therefore, the fuel cell is
capable of instantaneous power output.
(3) Oxygen Flow
FIG. 4c illustrates the oxygen flow when the regenerative fuel cell
system runs in the electrolyzer mode. As is known in the art,
oxygen is the product of the electrolysis reaction. During
electrolyzer operation, as water flows across the flow fields 35 of
the electrolyzer cells, oxygen is generated on the anodes of the
electrolyzer cells. The generated oxygen flows across the anode
flow field 35 of each electrolyzer cell together with the unreacted
water (FIG. 5a) and reaches the water outlet of the flow field 35
adjacent one end of each anode bipolar plate 31. Then the oxygen
exits the flow fields 35 of the electrolyzer cells via the water
outlets and leaves the electrolyzer portion 30 via the water outlet
63 provided on the electrolyzer end plate 70. The oxygen and water
preferably goes to a liquid-gas separator (not shown) where the
oxygen is separated from water, and the oxygen then goes to an
oxygen storage means from which it can be supplied to the fuel
cells during fuel cell operation. It should be mentioned that
during the electrolyzer operation, the oxygen does not enter the
fuel cell portion 20.
As will be understood by those skilled in the art, the fuel cell
and electrolyzer portions 20, 30 of the present system are not
necessarily arranged in the relation as disclosed in the first
embodiment described above. However, the positioning of the
electrolyzer portion 30 on top of the fuel cell portion renders a
number of advantages. Firstly, when the regenerative fuel cell
system is operating in fuel cell mode, water flows across the
membrane to humidify the hydrogen in the electrolyzer portion 20.
Any excess water tending to flood the hydrogen stream will be
carried out the bottom of the stack by the hydrogen flow. In
addition, water generated in the fuel cell portion in the fuel cell
mode will be carried immediately out of the stack in a downward
fashion to prevent water flooding in the fuel cells. Moreover, when
the system is operating in electrolyzer mode, oxygen evolved at the
anodes of the electrolyzer cells will naturally tend to flow
upwards out of the water/oxygen outlet 63 on the electrolyzer end
plate 70, preventing the trapping of O.sub.2 gas in the stack.
Reference will now be made to FIGS. 5a and 5b, which show
respectively the operation of the regenerative fuel cell apparatus
of the present invention in a fuel cell mode and an electrolyzer
mode. For simplicity and clarity, in FIG. 5, the fuel cell and
electrolyzer portions 20, 30 are shown schematically, to indicate
flows and to indicate flows between different sections of he
apparatus. Additionally, in FIG. 5, the bipolar plates 21, 22 and
31, 32 are indicated, for one cell in each of the fuel cell portion
20 and the electrolyzer portion 30. Also, while the plates 21, 22,
31, 32 are indicated as being bipolar in the sense that, in general
each plate will abut a plate of the opposite polarity in an
adjacent cell, the plates, 21, 22, 31, 32 can more generally be
considered as flow field plates.
To clarify the flow regime, the flows are shown passing directly
between the individual anodes and cathodes of the cells shown. It
will be understood that this is merely schematic. The actual
physical construction is as shown in FIG. 2, with the various cells
stacked together, and not side by side in the schematic
representation of FIG. 5; rather, it is ducts extending through the
fuel cell and electrolyzer portions 20, 30 that are in alignment
with one another. These ducts are also perpendicular to the plane
of the fuel cells.
It will also be understood that, in that manner, the fuel cells are
configured to provide chambers between adjacent fuel cells, and
also usually a corresponding chamber at the end of each stack.
These chambers provide a gas bypass in the electrolyzer portion and
are designated at 140; in a fuel cell portion, an exemplary chamber
is indicated at 142, and provides a coolant channel for water as a
coolant.
In the fuel cell mode of operation, water flows through the coolant
chambers 142 and then into the anode of the electrolyzer portion,
as indicated by the arrows 144. At the same time, air is supplied
through the electrolyzer portion, passing through the gas bypass
chamber 140 into the cathode of the fuel cell, as indicated by the
arrows 146. Hydrogen is also passed through the electrolyzer
portion 30. The hydrogen flows through the cathode of the
electrolyzer portion and then into the anode of the fuel cell
portion, as indicated by the arrows 148. Passing hydrogen and air,
or other oxidant, through the electrolyzer portion 30 can enable
some heat exchange to occur, as the electrolyzer portion 30 is
heated by the cooling water exhausted from the fuel cell portion
20. Additionally, some humidification of the hydrogen stream can
occur.
In the electrolyzer mode of operation, water again flows through
the coolant chambers 142 and then through the electrolyzer cell
anode as indicated by arrows 150. If desired, an air flow can be
provided as indicated at 152 through the gas bypass chamber 140 and
the fuel cell cathode, although this is not essential. As indicated
at 154, due to electrolysis, water leaving the electrolyzer portion
30 contains entrained oxygen. Hydrogen is generated in the cathode
of the electrolyzer portion. The hydrogen can be withdrawn from the
electrolyzer portion in a number of different ways. As indicated by
the arrow 156, the hydrogen can be taken out through the fuel cell
portion; however, as indicated in FIG. 4b, this is preferably
through a duct bypassing the actual flow fields of the fuel cell.
Alternatively, as detailed below in relation to FIG. 8, hydrogen
can be withdrawn from a side port in the separator plate 40,
between the electrolyzer and fuel cell portions, as indicated
schematically by the arrow 158. A further alternative is to take
the hydrogen out from the top of the electrolyzer portion 30,
indicated symbolically by the arrow 160; this is detailed below in
relation to FIG. 9, and does require provision for draining water
from the electrolyzer cathode.
Now referring to FIG. 6, a second embodiment of the present
invention is shown. For simplicity, the elements in the system that
are identical or similar to those in the first embodiment are
indicated with same reference numbers and for brevity, the
description of these elements is not repeated. In this embodiment,
the separator plate 40 and the insulator plates of the electrolyzer
and fuel cell portions that abut against the separator plate 40 are
omitted. The fuel cell portion 20 and the electrolyzer portion 30
are stacked together with only one insulator plate 28' sandwiched
between the two portions. The insulator plate 28' functions as both
the separator plate and the insulator plate for both portions. The
insulator plate 28' has three ports on it, i.e. the hydrogen port
41', the air port 42' and the water port 43', to communicate water
and process gases between the two portions. The principle of
operation in this embodiment is same as that in the first
embodiment. Therefore, it is not described in detail herein. In
this embodiment, the number of elements in the system is further
reduced, and the fuel cell portion 20 and the electrolyzer portion
30 are clamped together as a single unit.
Now, referring to FIG. 7, a third embodiment of the present
invention is shown. As for FIG. 6, identical or similar elements
are still indicated with same reference numbers and their
description is not repeated. In this embodiment, compared with the
first embodiment, the separator plate 40 and the two insulator
plates 28, 38 are removed. As compared to FIG. 6, the central
insulator plate 28 and separate current collector plates 27, 37 are
also removed. Instead, a single, common current collector plate
27', interchangeable between the outermost fuel cell anode and the
outermost electrolyzer cathode, is provided. Because the outermost
anode of the fuel cells is the negative electrode in fuel cell
stack and the outermost cathode of the electrolyzer cells is the
negative electrode in electrolyzer cell portion, and further
because the PEM in both electrolyzer cells and fuel cells are not
electrically conductive, it is possible to electrically connect the
anode of the fuel cell portion and the cathode of the electrolyzer
portion and run the system in either mode. It is to be understood
that in the first two embodiments above, the orientation of anode
and cathode bipolar plates of either the fuel cells or the
electrolyzer cells does not affect the feasibility of the present
invention; however in this third embodiment, the fuel cell portion
20 and the electrolyzer portion 30 have to be disposed such that
the current collector plate 27' is in contact with the anode
bipolar plate 21 of a fuel cell on one side and the cathode bipolar
plate 31 on the other side. Alternatively, as may be required in
some applications, the anode of the fuel cell portion 20 and the
cathode of the electrolyzer portion are both grounded. But this is
not always required. In the same way as the separator plate 40, the
common current collector plate 27' is provided with a hydrogen
port, an air port and a water port to communicate fluids between
the two portions. Accordingly, the size and weight of the
regenerative fuel cell system is further reduced in this
embodiment.
As another embodiment, the separator plate 40 in the first
embodiment or the insulator plate 28' in the second embodiment is
preferably provided with a valve, e.g. a ball valve 90 as shown in
FIG. 8a and 8b, and this valve 90 can be internal within the
separator plate 40. Here, the hydrogen port 41 is show having a
first hydrogen port 41a in communication with the cathode outlets
of the cathode flow fields 35 of the electrolyzer cells and a
second hydrogen port 41b in communication with the anode inlets of
the anode flow fields 25 of the fuel cells. Then, the all valve 90
is shown connected to first and second hydrogen ports 41a, 41b and
also to a line 92 connected to a hydrogen storage tank. In the
electrolyzer mode of the regenerative fuel cell system, the valve
90 may be in a first position to permit the electrolyzed hydrogen
to flow from the electrolyzer portion 30, i.e. from the first
hydrogen port 41, to an external hydrogen storage device. When the
system switches from electrolyzer peration to fuel cell operation,
the valve 90 switches to a second position, permitting hydrogen to
flow between the first and second hydrogen ports 41a, b, i.e. to
flow from the electrolyzer portion 30 to the fuel cell portion 20
in the same way described above. As known in the art, in the
electrolyzer portion 30, water is carried across the membranes from
the anode to the cathode during electrolysis, and is subsequently
entrained in the cathode flow fields by the evolved hydrogen. This
port arrangement prevents the water entrained in the generated
hydrogen from entering the fuel cell portion 20 and hence flooding
the fuel cells. Known separators and the like can be used to
separate water out, upstream of a hydrogen storage tank or the
like. As can be appreciated by those skilled in the art, the switch
mechanism may alternatively be provided internally of the separator
plate 40, namely in the hydrogen port 41.
Another way of prevent the fuel cell portion 20 from being flooded
during electrolyzer mode to generate hydrogen is shown in FIG. 9.
In this embodiment, hydrogen generated in the electrolyzer
operation is exhausted out of the electrolyzer portion 30 from the
port provided on the electrolyzer end plate 70. This port may be
the port 60, or a port additional to the fuel cell hydrogen inlet
60, the air inlet 62 and the water outlet 63. The hydrogen is
exhausted to an external storage device. As the electrolyzer
reaction proceeds, water accumulates on the bottom of the
electrolyzer portion 30. A purge valve 95 and a switch means, e.g.
a ball valve 96 may be provided on the bottom part of the
electrolyzer portion, in the embodiment, the separator plate 40 or
insulator plate 28'. The purge valve 95 maybe disposed in the
hydrogen port 41 and in communication with cathode outlets of the
cathode flow field 35 of the electrolyzer cells. During
electrolyzer operation of the system, the purge valve 95 and the
ball valve 96 are in closed position, thereby closing the hydrogen
passage between the electrolyzer portion 30 and the fuel cell
portion 20. Periodically, the purge valve 95 is opened to purge the
water out of the system to an external exhaust or water storage
device. When the system switches from the electrolyzer operation to
fuel cell operation, the purge valve 95 will be closed and the ball
valve will switch to open position that allows hydrogen flows from
the electrolyzer portion 30 to the fuel cell portion 20.
It should be appreciated that the spirit of the present invention
is concerned with the novel structure of the regenerative fuel cell
system and the unique usage of the electrolyzer as a humidifier
and/or a heat exchanger for the fuel cell. The internal structure
of the fuel cell portion and the electrolyzer portion does not
affect the design of the present invention. In other words, the
present invention is applicable to various types of fuel cells and
electrolyzers. In applications where fuel gas is not pure hydrogen,
reformers may be added before the inlet of the fuel gas for the
fuel cell portion. In this case, the structure of the present
system does not need to be changed.
It is to be anticipated that those having ordinary skill in this
art can make various modification to the embodiments disclosed
herein. For example, the shape of the fuel cells, electrolyzer
cells or the entire system might be varied, the electrolyzer and
the fuel cell portion might not necessarily be stacked one on top
of the other. But rather they can be in juxtaposed position or even
connected via conduits as needed in the situation. However, these
modifications should be considered to fall under the protection
scope of the invention as defined in the following claims.
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